SAADC using local timer

[NewtoM]

Hello,

based on the saadc example of SDK 11 I am trying to change the code to use the SAADC's local timer to sample the input given number of times (10 times for the below example), at a rate of 8000 Hz. Based on the documentation, it seems to be feasible by setting the NRF_SAADC->SAMPLERATE register's CC bitfield to 2000, plus, setting the MODE bit of the same register to 1.

The below code compiles and runs, but no values get written into sampleBuffer. Here is the code:

int main(void)
{
    uart_config();
    printf("SAADC test.\r\n");

    int32_t err_code;
    uint32_t const numberOfSamples= 10;
    int16_t sampleBuffer[numberOfSamples]= {0,1,2,3,4,5,6,7,8,9};

    nrf_saadc_channel_config_t channel_config=
    {
      .resistor_p = NRF_SAADC_RESISTOR_DISABLED,
      .resistor_n = NRF_SAADC_RESISTOR_DISABLED,
      .gain       = NRF_SAADC_GAIN1_6,
      .reference  = NRF_SAADC_REFERENCE_INTERNAL,
      .acq_time   = NRF_SAADC_ACQTIME_40US,
      .mode       = NRF_SAADC_MODE_SINGLE_ENDED,
      .pin_p      = NRF_SAADC_INPUT_AIN0,
      .pin_n      = NRF_SAADC_INPUT_DISABLED  
    };

    nrf_drv_saadc_config_t saadc_config=
    {
      .resolution= NRF_SAADC_RESOLUTION_10BIT,
      .oversample= NRF_SAADC_OVERSAMPLE_DISABLED,
      .interrupt_priority= APP_IRQ_PRIORITY_LOW
    };

    err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_config);
    APP_ERROR_CHECK(err_code);    

    uint32_t samplingRate= 8000;
    NRF_SAADC->SAMPLERATE = (16000000/samplingRate);      // [80..2047] Capture and compare value. Sample rate is 16 MHz/CC
    NRF_SAADC->SAMPLERATE|= SAADC_SAMPLERATE_MODE_Timers  // Rate is controlled from local timer (use CC to control the rate)
                        << SAADC_SAMPLERATE_MODE_Pos;

    NRF_SAADC->RESULT.PTR = (uint32_t)sampleBuffer;
    NRF_SAADC->RESULT.MAXCNT = numberOfSamples;    

    printf("Sampling ... ");
    nrf_saadc_task_trigger(NRF_SAADC_TASK_START);

    // while (nrf_saadc_event_check(NRF_SAADC_EVENT_END) != 0);
    nrf_delay_ms(100);

    printf("ended.\r\n");

    for (uint32_t i=0; i<numberOfSamples; ++i) { printf("%d,", sampleBuffer[i]); }

    while(1) { __WFE(); }
}

And here is the output on the UART terminal:

SAADC test.
Sampling ... ended.
0,1,2,3,4,5,6,7,8,9,

So it is easy to see that sampleBuffer still contains the numbers stored in it at the startup. Any help on how to make the code work would be appreciated.

Thanks! Tamas

[NewtoM]

I have some progress. I have just noticed that if after triggering the START task I also trigger the SAMPLE task, the values in my target array are indeed overwritten by some new values. So is this the way?

nrf_saadc_task_trigger(NRF_SAADC_TASK_START);
nrf_saadc_task_trigger(NRF_SAADC_TASK_SAMPLE);

I can't find in the documentation why both tasks should be triggered.

[ovrebekk]

Hi

Starting the SAADC module will only set up the DMA interface and prepare the ADC for sampling. To start the actual sampling you have to trigger the SAMPLE task as well.

Often these two activities are handled separately. You have to start the ADC to change the RAM buffer, while sampling typically occurs more often.

Best regards
Torbjørn

[NewtoM]

Thank you! I hope to see some local timer example, too, in one of the upcoming SDK versions. :) Tamas